The Obama administration "must declassify" a secret court order that forced Yahoo to scan the emails of its customers' emails as part of a surveillance program.
The order, first disclosed by Reuters last week, directed the company "to siphon off messages containing the character string the spies sought and store them for remote retrieval" by the intelligence community.
But questions remain over exactly how the intelligence community compelled the web giant to carry out the classified order -- specifically if Yahoo was forced to rewrite software at the behest of the order, or use an existing system designed to filter spam, as reported by The New York Times.
California-based rights group the Electronic Frontier Foundation said in a blog post that provisions passed by Congress in the wake of the Edward Snowden revelations "require" the government to review the order for potential public release.
Ordinarily, secret orders from the Foreign Intelligence Surveillance Court, which authorizes the government's surveillance requests, are never released.
But Congress last year passed provisions in the Freedom Act, which replaces parts of the expired Patriot Act, after the government was found to have widely interpreted the law in secret to carry out mass surveillance.
"If the reports about the Yahoo order are accurate -- including requiring the company to custom build new software to accomplish the scanning -- it's hard to imagine a better candidate for declassification and disclosure under Section 402," said Aaron Mackey, a legal fellow at the EFF.
"Given the divergent media reports about what the FISC required Yahoo to do, it is crucial for the public to see the order," he said.
Sen. Ron Wyden (D-OR), a staunch privacy advocate and member of the Senate Intelligence Committee, said in a brief statement that the alleged Yahoo order was "surprising and troubling".
"The USA Freedom Act requires the executive branch to declassify Foreign Intelligence Surveillance Court opinions that involve novel interpretations of laws or the Constitution and I certainly expect the Executive Branch to follow this law."
The EFF earlier this year brought a case against the US government over secret orders that require tech companies to decrypt user communications.
The suit follows a ZDNet report that the government was secretly demanding source code from Silicon Valley.
"Releasing the Yahoo opinion will help us begin to answer the bigger questions about the Yahoo order and its dubious constitutionality," said Mackey. "Releasing all significant FISC opinions will not only comply with what Congress required under [the Freedom Act], it will help us better understand exactly what the FISC has secretly decided about our civil liberties."